1. Field of the Invention
The present invention is directed to an electrically conductive polymer film to be used in potentiometers or similar electric and electronic applications where a moving or movable wiper contacts the polymer film. More specifically, the present invention is directed to an electrically conductive polymer film, usable in potentiometers and the like, having a plurality of particles admixed with the film which protrude from the surface of the film and render it uneven on a micro scale.
2. Brief Description of the Prior Art
Potentiometers and like electric devices where electric resistance is varied depending on the position of a contact wiper moving on the surface of an electrically conductive resistor layer, are old in the art. It is also old and well established in the art to form the electrically conductive resistive layer from a cured polymer film which contains sufficient amount of an electrically conductive material (such as carbon, graphite or metal powder) to make the polymer film conductive. Such state-of-the-art electrically conductive polymer films are typically deposited on ceramic, glass, plastic or paper-type substrates in an uncured form, and are typically cured thermally to provide a "thick" film of approximately 5 to 25 microns thickness. The cured thick film of the resistor is contacted in the potentiometer by a wiper contact, which in many applications moves (cycles) continuously, and sometimes rapidly. Ideally, the potentiometer would be expected to function properly and retain its original electric resistance characteristics even after several million cycles. However, until the present invention, this goal has been rather unattainable in the prior art.
The following United States patents describe or relate to the above-summarized technology: U.S. Pat. Nos. 3,299,389, 3,329,920, 4,100,525, 4,350,741, 4,694,272 and 4,728,755.
As it was noted above, a significant disadvantage of prior art polymeric thick films for potentiometer applications lies in the fact that the electric resistive characteristics of the device change during the life of the device and results in more or less resistive "noise". This is best explained with reference to FIG. 4 of the drawings appended to the present application for United States Letters Patent, where the surface profile (as measured by a profilometer) of a conductive thick film of a prior art potentiometer is shown. As can be seen, the surface is not absolutely even, rather it has "hills" and "valleys" which tend to be eroded and leveled during prolonged cycling of a contact wiper over the surface. The problem in this regard is that, as experience has shown, erosion of the uneven polymer film surface results in formation and accumulation of debris which is deposited particularly in the wiper turn-around locations. Other problems related to wear of the film and the wiper also occur, these include surface cracking or peeling. Generally speaking, it was thought in the prior art that a solution to the above-noted problems is to make the surface of the electrically conductive thick film of the potentiometer smoother and more even, and efforts were made in this regard.
Nevertheless, formation and accumulation of debris and related phenomena in prior art potentiometers (and like instruments) having conductive polymeric thick film resistors and moving contact wipers have presented a major problem in the art, and rendered it difficult or unattainable to fabricate potentiometers which had substantially unimpaired performance even after approximately one million cycles or prolonged dithering of the contact wiper. The present invention provides a solution to this problem, and provides an electrically conductive polymeric thick film for application in potentiometers and like devices which function, without significant degradation in performance, for several million cycles.